1. Field of the Invention
The present invention relates to a transfer device, an image forming apparatus, and a power supply control method.
2. Description of the Related Art
Typically, electrophotography image forming apparatuses apply a direct-current (DC) voltage to an electrostatic toner pattern formed on an image carrier, thereby moving a developer, such as a toner, forming the electrostatic toner pattern to a sheet. Thus, electrophotography image forming apparatuses transfer the electrostatic toner pattern onto the sheet.
In use of a sheet having a highly uneven surface and low surface smoothness, such as leather-like paper and Japanese paper, a developer is less likely to be transferred onto recessed portions compared with protruding portions. This renders printing on the recessed portions unclear.
To address this, Japanese Laid-open Patent Publication No. 2008-058585, for example, discloses a technology for increasing the transfer ratio of a developer onto recessed portions by superimposing an alternating-current (AC) voltage on a DC voltage for transfer to generate a sinusoidal wave and causing the developer to oscillate.
In the conventional technology, a toner reciprocates between a toner carrier and a sheet with the AC frequency. This increases the transferability at the recessed portions on the sheet surface. However, the developer scatters due to the oscillation of the toner, thereby generating a blur on an image. In the conventional technology, even if the voltage is output by superimposing the AC component for oscillation of the toner on the DC component for transfer, the superimposition makes the peak voltage in a transfer-direction polarity extremely high depending on conditions for image formation. This facilitates aerial discharge, thereby generating a void at the protruding portions on the sheet surface. To address this, it is necessary to develop a technology for increasing the transfer ratio of the developer onto the recessed portions on the sheet surface and forming a high-quality image.
Therefore, there is a need for a transfer device, an image forming apparatus, and a power supply control method that are capable of increasing the transfer ratio of a developer onto recessed portions on a sheet surface and improving the image quality regardless of conditions for image formation.